Employee Benefit Plans (Estimated Fair Value on a Recurring Basis) (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 8,948	$ 9,688	$ 9,009
Pension Benefits | Fixed Maturity Securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	7,301	7,891
Pension Benefits | Corporate fixed maturity securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	3,351	3,834
Pension Benefits | U.S. government bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,785	1,784
Pension Benefits | Foreign government
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	837	1,037
Pension Benefits | Federal agencies
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	88	169
Pension Benefits | Municipals
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	240	335
Pension Benefits | Short-term Investments
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	199	327
Pension Benefits | Other (1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	801	405
Pension Benefits | Equity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	901	977
Pension Benefits | Other investments
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	708	785
Pension Benefits | Derivative assets
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	38	35
Pension Benefits | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	2,284	2,282
Pension Benefits | Level 1 | Fixed Maturity Securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,525	1,433
Pension Benefits | Level 1 | Corporate fixed maturity securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Pension Benefits | Level 1 | U.S. government bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,314	1,256
Pension Benefits | Level 1 | Foreign government
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Pension Benefits | Level 1 | Federal agencies
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	35
Pension Benefits | Level 1 | Municipals
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Pension Benefits | Level 1 | Short-term Investments
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1	135
Pension Benefits | Level 1 | Other (1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	210	7
Pension Benefits | Level 1 | Equity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	706	797
Pension Benefits | Level 1 | Other investments
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	20	19
Pension Benefits | Level 1 | Derivative assets
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	33	33
Pension Benefits | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	5,973	6,770
Pension Benefits | Level 2 | Fixed Maturity Securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	5,774	6,447
Pension Benefits | Level 2 | Corporate fixed maturity securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	3,350	3,833
Pension Benefits | Level 2 | U.S. government bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	471	528
Pension Benefits | Level 2 | Foreign government
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	837	1,037
Pension Benefits | Level 2 | Federal agencies
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	88	134
Pension Benefits | Level 2 | Municipals
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	240	335
Pension Benefits | Level 2 | Short-term Investments
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	198	192
Pension Benefits | Level 2 | Other (1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	590	388
Pension Benefits | Level 2 | Equity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	195	177
Pension Benefits | Level 2 | Other investments
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	144
Pension Benefits | Level 2 | Derivative assets
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	4	2
Pension Benefits | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	691	636
Pension Benefits | Level 3 | Fixed Maturity Securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	2	11
Pension Benefits | Level 3 | Corporate fixed maturity securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1	1	0
Pension Benefits | Level 3 | U.S. government bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Pension Benefits | Level 3 | Foreign government
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Pension Benefits | Level 3 | Federal agencies
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Pension Benefits | Level 3 | Municipals
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Pension Benefits | Level 3 | Short-term Investments
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Pension Benefits | Level 3 | Other (1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1	10	9
Pension Benefits | Level 3 | Equity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	3	0
Pension Benefits | Level 3 | Other investments
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	688	622	637
Pension Benefits | Level 3 | Derivative assets
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1	0	65
Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,360	1,434	$ 1,386
Other Postretirement Benefits | Fixed Maturity Securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,186	1,270
Other Postretirement Benefits | Corporate fixed maturity securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	313	382
Other Postretirement Benefits | U.S. government bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	268	275
Other Postretirement Benefits | Foreign government
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	90	102
Other Postretirement Benefits | Federal agencies
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	16	17
Other Postretirement Benefits | Municipals
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	29	28
Other Postretirement Benefits | Short-term Investments
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	398	398
Other Postretirement Benefits | Other (1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	72	68
Other Postretirement Benefits | Equity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	173	154
Other Postretirement Benefits | Other investments
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	9
Other Postretirement Benefits | Derivative assets
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1	1
Other Postretirement Benefits | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	428	452
Other Postretirement Benefits | Level 1 | Fixed Maturity Securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	272	297
Other Postretirement Benefits | Level 1 | Corporate fixed maturity securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	20
Other Postretirement Benefits | Level 1 | U.S. government bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	268	269
Other Postretirement Benefits | Level 1 | Foreign government
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Other Postretirement Benefits | Level 1 | Federal agencies
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Other Postretirement Benefits | Level 1 | Municipals
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Other Postretirement Benefits | Level 1 | Short-term Investments
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1	8
Other Postretirement Benefits | Level 1 | Other (1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	3	0
Other Postretirement Benefits | Level 1 | Equity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	155	154
Other Postretirement Benefits | Level 1 | Other investments
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Other Postretirement Benefits | Level 1 | Derivative assets
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1	1
Other Postretirement Benefits | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	932	982
Other Postretirement Benefits | Level 2 | Fixed Maturity Securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	914	973
Other Postretirement Benefits | Level 2 | Corporate fixed maturity securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	313	362
Other Postretirement Benefits | Level 2 | U.S. government bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	6
Other Postretirement Benefits | Level 2 | Foreign government
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	90	102
Other Postretirement Benefits | Level 2 | Federal agencies
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	16	17
Other Postretirement Benefits | Level 2 | Municipals
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	29	28
Other Postretirement Benefits | Level 2 | Short-term Investments
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	397	390
Other Postretirement Benefits | Level 2 | Other (1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	69	68
Other Postretirement Benefits | Level 2 | Equity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	18	0
Other Postretirement Benefits | Level 2 | Other investments
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	9
Other Postretirement Benefits | Level 2 | Derivative assets
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Other Postretirement Benefits | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Other Postretirement Benefits | Level 3 | Fixed Maturity Securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Other Postretirement Benefits | Level 3 | Corporate fixed maturity securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Other Postretirement Benefits | Level 3 | U.S. government bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Other Postretirement Benefits | Level 3 | Foreign government
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Other Postretirement Benefits | Level 3 | Federal agencies
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Other Postretirement Benefits | Level 3 | Municipals
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Other Postretirement Benefits | Level 3 | Short-term Investments
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Other Postretirement Benefits | Level 3 | Other (1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Other Postretirement Benefits | Level 3 | Equity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Other Postretirement Benefits | Level 3 | Other investments
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Other Postretirement Benefits | Level 3 | Derivative assets
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 0	$ 0